The present invention relates generally to electrical microcircuit structures and to methods for making such structures. More particularly, the invention is concerned with the provision of silicon nitride-passivated hybrid circuits that permit post-passivation trimming of included thin-film circuit elements.
In the manufacture of thin-film and monolithic hybrid microcircuits, passive circuit elements--such as resistors and capacitors--are formed from films of materials only a few thousand angstroms thick. These films typically are deposited on a supporting substrate by vacuum evaporation or cathodic sputtering, with the required patterning being effected simultaneously or in a subsequent procedure. A protective overcoating or passivation film usually is applied to such circuits for environmental protection prior to final packaging, particularly if they will not be sealed within a hermetic enclosure. Silicon nitride (Si.sub.3 N.sub.4) has found increasing use as a passivation coating material because of its high resistivity and dielectric strength, excellent chemical resistance, and superior electrical and thermal stability.
The values of thin-film electrical components typically fall within a 5-25% tolerance range as fabricated, even with well-controlled processes. More precise values are required in many circuit applications, and in others it may be necessary to adjust component values on an individual basis to "custom-tune" a circuit. This is accomplished by a trimming operation in which portions of a component are physically removed. Airborne abrasive, electric arc and laser beam trimming systems have been developed for this purpose and are commercially available. Laser trimming systems have a number of significant advantages compared to the others, including better accuracy, much greater speed, and cleaner operation. A further important factor is the ability of laser systems to trim circuit components through an overlying passivation film if a laser operating in the visible or near-infrared region is used. This allows a circuit to be adjusted for optimum operation after its fabrication is essentially complete.
In the past it has not been possible to laser trim certain thin-film components in silicon nitride-passivated circuits without damaging the passivation layer. During the trimming of Nichrome and other nickel- or chromium-containing films, for example, voids and cracks in the silicon nitride layer are produced and form an entry point for moisture and contaminants. Because of the superior protection afforded by silicon nitride, there is a need to provide a Si.sub.3 N.sub.4 -protected microcircuit structure that permits post-passivation trimming of included thin-film components containing nickel or chromium. A related need is to provide a method for forming such structures on a variety of substrates.